Almost every modern electronic product, ranging from toys to massive computers, now uses integrated circuits (“ICs”). ICs are generally made using photolithographic processes that involve manufacturing a template containing patterns of the electrical circuit as transparent and opaque areas. The patterned template is referred to as a “reticle” or “mask”.
A radiation source, such as a light, is used to copy or “pattern” multiple images of the mask onto a photosensitive material, such as a photoresist, on the surface of a silicon wafer. Once features are patterned on the photoresist, further processing is performed to form various structures on the silicon wafer. The completed wafer is then cut (or “diced”) to form the individual ICs.
Engineers typically use computer-aided design (“CAD”) to create a schematic design of the mask. One technique, Levenson Phase-Shifting, also known as Alternating Aperture Phase-Shifting, is used to create small features on ICs. Such small features are generated by a pair of areas in the mask called shifters.
An alternating element phase-shift mask (“PSM”) normally includes a substantially transparent substrate composed, for example, of quartz. Phase-shifting material is situated on the mask substrate to provide a phase shift to light radiation as it passes through the mask. The phase-shifting material may be, for example, silicon nitride or other suitable transparent material such as oxides or oxynitrides. In such an alternating element PSM, discrete non-phase-shifting components are disposed alternatively adjacent to discrete phase-shifting components.
An attenuated PSM contains discrete layers of absorbers, composed for example of chromium, disposed on the mask substrate.
Hybrid attenuated-unattenuated PSMs can also be combined with alternate element PSMs.
Shifters change the phase of the light passing through them. Two shifters can be used on a mask to shine light on the same region of a photoresist. In a region where the light passing through one of the shifters is in phase with the light passing through the other shifter, a feature can be created on the photoresist that is narrower than the distance between the two shifters. By reducing the distance between the two shifters, very small features can be created on the photoresist. The width of the feature can be considerably less than could be produced by the same optical system without phase shifting.
Recent developments in PSM technology have been focused on single exposure chromeless phase lithography (“CPL”), also known as phase-edge chromeless off-axis illumination (“PCO”). While such a technique may be appropriate for making IC line and space structures, it is generally unsatisfactory for forming contact hole patterns. This is due in part to the different pattern designs needed for isolated contact, semi-dense contact, and dense array contact configurations. Randomly distributed contacts pose an additional problem because they do not fit into any of these other three types.
Thus, a need still remains for a unit cell CPL contact hole structure wherein the identical unit cell can be used in any contact arrangement, thereby enabling both randomly scattered and neatly arranged contact holes to be phase shifted using the same unit cell structure. A need also remains for improvements to the process margins. One such improvement would provide, for example, for increasing the intensity contrast of such a unit cell compared to attenuated phase-shifting mask contact techniques.
In view of the ever-increasing need to save costs and improve efficiencies, it is more and more critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.